This invention relates to a novel bonding mechanism to ensure tight adhesion between a chemically deposited metal layer and an insulating substrate and provides, in particular, a printed circuit board which is excellent in heat resistance. It also relates to a method for making the same.
A variety of methods have heretofore been developed for the chemical metal plating over insulating substrates. For instance, there has been known a method for plating a metal over a plastic material, which comprises etching a shaped article of an acrylonitrile-butadiene-styrene copolymer with a chromic acid-sulfuric acid solution to rupture the double bonds of the butadiene, thus forming polar groups such as carbonyl and carboxyl and, at the same time, fine pores as a result of removal of butadiene by dissolution, then subjecting the surface of the thus treated article to an sensitizing treatment using stannous chloride and palladium chloride which have a catalytic effect on chemical plating, and thereafter chemically depositing a metal layer such as nickel or copper on the activated surface of the article. This method aims at taking advantages of both the chemical bonding due to the polar groups and the mechanical bonding due to the fine pores in establishing tight adhesion between the metal and the plastic material.
In the field of electronic equipments, on the other hand, so-called printed circuit boards made by forming printed circuits on insulating substrates made of phenolic resins, epoxy resins, and polyester resins as major materials have played an important role in miniaturization and weight reduction of these electronic equipments.
One of the known effective means for making such printed circuit boards is the additive method which employs a technique for plating a metal on plastic materials and comprises the following sequence of steps: as shown in FIG. 1, an insulating substrate 1 is coated on the surface with a thermosetting adhesive 2, such as a nitrile-rubber-modified phenolic resin; the adhesive 2 is cured partially or entirely; the surface of the adhesive is etched with a chromic acid-sulfuric acid solution to provide finely distributed minute pores or depressions 3; the adhesive layer is then treated with an aqueous solution containing palladium chloride to sensitize the adhesive layer on the surface; the thus treated adhesive surface is subjected to chemical plating to deposit a metal layer 4; a negative-image resist pattern is applied onto the metal layer 4; electroplating is applied to grow a metal layer on the said chemically deposited metal layer 4 exposed in prescribed pattern; the resist layer is removed; the chemically deposited metal in the undesired areas is removed by etching; and, if necessary, the adhesive layer is ultimately cured to bond the metal layer firmly to the substrate. In the above method, firm bonding of the metal layer 4 to the insulating substrate 1 is obtained, similarly to the case of the metal plating on plastic materials, by surface etching of an adhesive layer, which gives rise to chemical adhesion caused by polar groups such as carbonyl and carboxyl which are formed and also to mechanical adhesion owing to the anchor effect which is attributable to the minute pores or depressions 3.
However, when such a method is employed in making a printed circuit board, the chromic acid-sulfuric acid solution used to form minute pores 3 by etching tends to remain in the minute pores. In some cases, the minute pores contain also some amount of undesirable chemicals which are used in the steps subsequent to the etching. Consequently, when such a printed circuit board is included in an electronic equipment, there is a danger of electrolytic corrosion caused by the electric charge. The printed circuit board made by the above-noted method has also another disadvantage of inferior soldering resistance, because such a printed circuit board is liable to absorb water during electroless or electrolytic plating owing to the presence of minute pores 3 on the surface of adhesive layer 2 and the absorbed water is released as a vapor on sudden heating during soldering work to mount various components to the printed circuit board, thus resulting in blistering due to the released vapor between the insulating substrate 1 and the metal layer 4.